The cutting insert used in milling cutter tools consists generally of a prismatic body having a planar base from which extend side surfaces--relief flank surfaces--which are intersected by the upper surface--the cutting rake surface. The cutting insert has at least one cutting edge which is defined as an edge created by the intersection of the cutting rake surface and the relief flank.
The insert is so held in the tool that the cutting edge is located in the circular cutting path of the tool and is inclined with reference to the rotary axis by an angle referred to as the axial rake angle.
The magnitude of the axial rake angle has a direct influence on the stability of the tool during operation and also on the durability of the cutting edge. In general, it is advantageous to have the inserts disposed at large axial rake angles. With tools having interchangeable inserts, providing for such large axial rake angles gives rise to a structural weakening of the tool. In consequence, the most common milling cutter tools are constructed for use with relatively low axial rake angles.
In addition to the axial rake, another characteristic of the insert which is of considerable significance on the operational efficiency of the tool is the relief angle of the insert. As used in the specification, the term "relief angle" is the angle subtended between the relief flank of the insert when mounted in the milling cutter, and the tangent of the circular path defined by a point on the cutting edge. The term "relief flank angle", on the other hand, is the angle defined between the relief flank of the insert and a line normal to the base of the insert.
It is known that with inserts for milling cutter tools the relief angles employed are relatively large as compared with the angles used with stationary cutting tools, and this is particularly the case with milling tools having low diameter cutting paths. Increasing the relief angle leads to a weakening of the cutting edge and there is in consequence a limitation of the magnitude of the relief angle. The magnitude of the relief angle is determined in accordance with the material of the work piece, the material from which the cutting edge is formed and the cutting path diameter of the tool. In general, with hard and tough work piece materials, the relief angle is between 6.degree. to 8.degree. with tools of high speed steel, and between 5.degree. to 7.degree. with carbide tools. On the other hand, with work pieces of mild steels, cast iron, etc., the relief angle can be as high as 20.degree..
As far as the relief flank angle is concerned, these are generally chosen to be in the general range of 15.degree. to 20.degree. for relatively soft materials and in the general range of 11.degree. to 15.degree. for relatively hard materials.
Various design and operational considerations indicate the use of inserts with flank relief angles which are in the higher regions of the range. In this connection, however, the higher the flank relief angle associated with the cutting edge of the insert, the greater the degree of unsupported overhang of the relief flank vis-a-vis the tool seating. Furthermore, it will readily be appreciated that serious difficulties arise when, for example, it is desired to securely locate an insert having a relatively high relief flank angle in a cutter tool seating designed to accommodate an insert having a lower relief flank angle.